dumb noob question: how do you know when to use 11.1 or 7.2 v? - RC Groups
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May 22, 2005, 01:33 AM
Old Tyme Electric
steve crewdog's Avatar

dumb noob question: how do you know when to use 11.1 or 7.2 v?

I'm starting to get the hang of this electirc stuff then I got stopped in my tracks today. I was buying an axi 4120/18 for the Antic and the gent asked me if I was going to run the motor at 11.1 or 7.2 v so I could buy the batteries.

I gave him my favorite Blank Stare (I've gotten good at it since I've been making the transition from gas to electric.) and he figured it out for me. We're going to combine a Kokam 3 cell 20C 2100 mh 11.1 v and another 2 cell 2100 in series for the power. I sorta understood how he decided 11.1v but now I'm trying to remember and I'm having trouble.


Last edited by steve crewdog; May 22, 2005 at 01:49 AM.
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May 22, 2005, 01:46 AM
Registered User
ryanl2006's Avatar
If you connect them both in series then it will be a 5s total and 18.5 volts.

You use 3s to have more voltage less amps and/or a smaller prop. 2s gives you higher amps per watt, and generally uses a bigger prop. Those are very general statetements, it really all depends on what you want to do and your plane/motor/esc/battery setup.

I hope this post did not confuse you any more.

May 22, 2005, 03:30 AM
Registered User
LIke ryanl2006 says, put a 3-cell and a 2-cell in series and you've got a 5-cell pack of about 18.5v (5 x 3.7v).

Looking up the Axi 4120/18 spec I see that it's designed for "16-20 cells", by which they mean NiCad or NiMh cells at about 1.2v per cell. So that's about 19.2 to 24v. So I would say that six Lipos would be bang in the middle of the recommended range.

But that's not all there is to it: You need to know what power (watts) your model needs (watts = volts x amps); what pitch of prop will drive your plane best for your style of flying; then what size prop will result in the right number of amps. Hobby Lobby http://www.hobby-lobby.com/brushless-axi4120.htm recommends 13x6.5 to 14x7 prop. They also suggest it will be drawing 39.7amps, so I would think that your 2100 packs (theoretically 42amps if the 20C rating is believable) might be marginal.

I've converted 2 glow models to electric using that sequence of logic, plus help from this forum, so I'm no expert. I'm sure others in this forum will be able to be more specific concerning your particular model.
May 22, 2005, 04:16 AM
Old Tyme Electric
steve crewdog's Avatar
Originally Posted by abenn
They also suggest it will be drawing 39.7amps, so I would think that your 2100 packs (theoretically 42amps if the 20C rating is believable) might be marginal.


I see the 26.4 amp for a 13x8, and I see the 37.9 amp for the 14x9.5, don't see the 39.7 amp.....

I must have forgotten that part, better go back over my notes in the morning. VoltsXamps=Watts. Watts/pound determine performance. Lower voltage could be made up with higher amperage and visaversa.

Got it.

Good info guys, Thanks and keep it coming.
Last edited by steve crewdog; May 22, 2005 at 04:21 AM.
May 22, 2005, 06:20 AM
Registered User
Quite right, too early in the morning for me -- I mis-read the 37.9

Anyway, the point I was making (and I'm sure you'll have read it elsewhere in this forum) is that some manufacturers are more optimistic than others in their C ratings, so their packs might not be able to deliver the amps that you think they would. I haven't used LiPos myself yet, so I'm not qualified to comment whether Kokam tell it like it is, or not.

Lower voltage can be compensated for by higher amps, and vice versa; and the size and pitch of prop determines the amps, if everything else is equal. But other things then come into play such as motor r.p.m. (depends on voltage, if everything else is equal), motor efficiency, and ESC rating. That's why you're best to start off within the manufacturer's recommended range unless you have advice from someone who's successfully used your motor with a specific battery/prop combo in a model similar to yours.

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